1. Field of the Invention
The present invention relates to an optical element and, particularly, to an optical element that is preferably used for transmitting and receiving optical information.
2. Description of the Related Art
Conventionally, as an optical element with wavelength-selectivity which is capable of reflecting or transmitting light by selecting wavelength of the light, there has been known an optical element comprising an optical filter that reflects or transmits light in accordance with wavelength of the light to be used.
Such optical element is used for a wavelength multiplex transmitting/receiving module and the like for separating or synthesizing light waves, which is mounted to a single-core bidirectional transmission system in an optical fiber communication system.
Examples of invention relating to such optical element are those described in Patent Literature 1 and Patent Literature 2.
These documents disclose inventions which provide effects such as reduction in the number of components, reduction in core adjusting steps, simplification of assembly, downsizing, reduction in cost, etc.
Conventionally, the above-described optical filter has been obtained by forming a dielectric multilayer film on a substrate.    [Patent Literature 1] Japanese Patent Unexamined Publication 2004-354752    [Patent Literature 2] Japanese Patent Unexamined Publication 2005-84188
In accordance with a recent spread of optical fiber communication technology and an increase in demand thereof, there are more and more requests to achieve reduction in the number of components, downsizing, low cost, and improved mass-productivity in an optical element that performs transmission and reception of optical information.
However, the inventions described in Patent Literatures 1 and 2 face a problem of productivity since the inventions require a complicated work wherein an optical filter and an optical block (housing or the like) for holding the optical filter are manufactured separately, and the optical filter is placed at a prescribed position of the optical block.
If an optical filter constituted of a multilayer film is formed on the surface of an optical block by a film-forming method, a work for placing the optical filter can be omitted. However, there is a multilayer film formed also on other surfaces of the optical block than the surface where the optical filter is to be formed, which may result in deterioration of the optical property of the optical element.
For example, if a multilayer film is formed on a lens face of an optical block that comprises the lens face on the incident end or emission end of light, there may hinder light incident on the lens face or light emission therefrom depending on the property of the multilayer film.
Furthermore, in order to carry out the work for placing the optical filter easily but properly, it is inevitable that the optical filter placing area of the optical block has a size of certain extent. Thus, it has been difficult to achieve more downsizing than that of the conventional cases.
Moreover, conventionally, an antireflection coating has been formed on the optical block for improving the optical property of the optical element. In the case of forming such antireflection coating, the number of components and cost is more increased, and mass-production becomes more difficult.
As a result, there is a conventional problem that it cannot sufficiently meet the demands for achieving more reduction in the number of components, downsizing, low cost, and an improvement in the mass-productivity.